Definitely length, weight isn't an issue so much (within reason), I've only got dinky wee arms you see, and the M4s I've used were just a little too big for my span, which takes it's toll after a day of play.
"Or why not just go for a P90, if that's what tickles your fancy." That comes down to budget and the advice of the last few folk I spoke to regarding P90s, they basically said the cheaper ones are pretty rough and that mags don't always work as they should and are more expensive in general, so if I were to get one I should go for the more expensive ones (£400 ish ones). Not 100% on that advice though as I'm new to airsofting so haven't really sussed out brands and such yet.